Use of time reversal focal laws is a known method of inspecting complex surfaces with a PA probe. See, for example, Beardsley, B. et al. 1995. ‘A Simple Scheme for Self-Focusing of an Array’, Journal of Nondestructive Evaluation, Vol. 14, No. 4 (1995), p 169-179 and ‘Time Reversal of Ultrasonic Fields—Part I: Basic Principles’, Mathias Fink, IEEE Transaction on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 39, No. 5, September 1992, p 555-566. The method involves creating an incident acoustic wave front which is parallel to the surface being inspected, the parallel wave front allowing improved resolution for imaging sub-surface flaws. The parallel wave front is iteratively created as the PA probe is scanned along the test surface, the iteration involving forming focal laws with a time-reversal algorithm which reverses the arrival time at the probe of acoustic echoes reflected from the test surface.
When the time-reversal algorithm is used, the focal laws are adapted at each probe scan position in order to create a wave front having normal incidence at every point on the surface. However, since the focal laws are changing continuously during the scan, any calibration procedure performed with a particular focal law is invalidated. In particular, the calibration gain, which defines the inspection sensitivity, is heavily dependent on the focal laws, and will therefore change in the course of the scan. The part coverage and the flaw sizing accuracy also depend on the focal laws, and will therefore also change in the course of the scan. There is no known method in existing practice to compensate for the changes in calibration gain, part coverage and flaw sizing accuracy.
Another problem with the time reversal algorithm is that the delays computed cannot be directly used to obtain the profile of the test surface because the wave front created by the focal laws is a combination of several wave fronts, namely the wave fronts of each of the transmitting elements. It is necessary to have detailed knowledge of the surface profile because it allows the user to ensure complete coverage of the test surface and to more precisely determine flaw size and position within the test object.